1. Field of the Invention
The present invention relates to an image processing method for processing a document image, an image processing apparatus, and a computer-readable medium.
2. Description of the Related Art
In recent years, progress has been made in converting information into electronic formats, and systems for scanning a paper document with a device such as a scanner and converting the document into an electronic format, instead of storing the document as-is, and for transmitting such electronic data to other devices are becoming increasingly popular.
Although electronic documents require sharpness for their character portions and uniform colors for their backgrounds in order to realize improved readability and increased compression rate, various types of noises, blurring, and irregularity can occur at the time of actually converting paper documents into electronic form.
Japanese Patent Laid-Open No. 2002-142128 has disclosed a method for, converting an input image into a low-resolution image, selecting regions of nearly uniform colors as candidates for regions of a representative color from the low-resolution image, and applying the candidates for the regions to a high-resolution image to decide a representative color and regions to be colored with the representative color, in order to simplify color-reduction processing.
Deciding a representative color in a low resolution image, as described in Japanese Patent Laid-Open No. 2002-142128, simplifies the process to decide a representative color since the number of original colors is reduced. However, in cases where a halftone dot region includes a character and the character is small in size, a minute region such as a portion corresponding to the dot of the letter “i” can be smoothed into a surrounding color at the time of low-resolution conversion and thereby can appear with a reduced frequency in the histogram. In such cases, if a representative color is decided by reference to only the frequencies in the histogram, there is the possibility of the color of the character not remaining as a representative color. That is, in such cases, the color of the character is lost at the time of quantization.